Acceleration (rate of change of velocity) is generally measured indirectly, by measuring the force exerted by, or restraints that are placed on, a reference mass to hold its position fixed in an accelerating body. Acceleration is computed using the relationship between restraint force and acceleration given by Newton's Second Law of Motion: the force is equal to the product of the mass and acceleration. Therefore, the precision by which acceleration can be determined is directly related to the precision by which force and mass can be measured.